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Excel water activity worksheet download pageThe following paragraphs describe any calculations that are done in non-empty white-colored cells of Sheet2.
IngrInitMoist (cells A4:A18) contain the initial moisture value for each ingredient in the mixture calculated from its moisture sorption isotherm equation. One equation is used per row. The value of the independent variable for each equation is taken from the corresponding InitIngrAw cell (Sheet1 cells D4:D18) that contains a guessed (before the FindAw macro is executed) or calculated (after the FindAw macro is executed) initial water activity (Aw) value.
AwStarGoal (cell B2) is an expression that the goal seeking procedure in the FindAw macro is trying to make equal to zero. When that occurs, AwStar, the equilibrium water activity of the mixture, has been found. The cell calculates the expression: AwStarGoal = IsoMixMoisture - MixInitMoistDryB
InitIngrAwGoal (cells B4:B18) are expressions that the goal seeking procedure in the FindAw macro is trying to make equal to zero. When that occurs, InitIngrAw, the initial water activity of each ingredient in the mixture, has been found. The expression is: InitIngrAwGoal = IngrInitMoist - InitMoistDryB
MixInitMoistDryB (cell C2) contains the total initial moisture content of the dry food mixture expressed in percent on a dry basis. The formula used is: MixInitMoistDryB = SumMeasIngrMoist / SumInitDryWt * 100
InitMoistDryB (cells C4:C18) convert entered wet basis initial moisture content to a dry basis for each ingredient. The dry basis moisture content is expressed in percent, as grams of water per 100 grams of dry matter. The formula is: InitMoistDryB = (100 * InitMoistWetB) / (100 - InitMoistWetB)
SumInitDryWt (cell D2) contains the total initial dry weight (expressed in grams, dry matter only) of the dry food mixture. It is calculated as follows: SumInitDryWt = SUM(D4:D18)
InitDryWt (cells D4:D18) convert entered wet weights to dry weights for each ingredient. The initial dry weight is expressed in grams (dry matter only). The formula used for the conversion is: InitDryWt = InitWetWt * ((100 - InitMoistWetB) / 100)
SumMeasIngrMoist (cell E2) contains the total measured initial moisture in the mixture (in grams). It is calculated as follows: SumMeasIngrMoist = SUM(E4:E18)
MeasIngrMoist (cells E4:E18) contain the measured initial moisture in each ingredient (in grams). They are calculated as follows: MeasIngrMoist = InitDryWt * InitMoistDryB / 100
SumIsoIngrMoist (cell F2) contains the the total equation-calculated moisture (in grams) in the entire dry food mixture. The value changes when the guessed AwStar value is updated to the calculated AwStar value. It is calculated as follows: SumIsoIngrMoist = SUM(F4:F18)
IsoIngrMoist (cells F4:F18) contain the amount of moisture in one ingredient (in grams) calculated from its moisture sorption isotherm equation. These values change when the guessed AwStar value is updated to the calculated AwStar value. They are calculated as follows: IsoIngrMoist = InitDryWt * IngrEqMoist / 100
IsoMixMoisture (cell G2) contains the total moisture of the dry food mixture (in percent on a dry basis, expressed as grams of water per 100 grams of dry matter). The value changes when the guessed AwStar value is updated to the calculated AwStar value. The formula is: IsoMixMoisture = SumIsoIngrMoist / SumInitDryWt * 100
IngrChangeMoist (cells G4:G18) contains the change in its ingredient's dry basis moisture value (in percent on a dry basis, expressed as grams of water per 100 grams of dry matter). The result is positive if moisture was gained and negative if it was lost. It is calculated as follows: IngrChangeMoist = IngrEqMoist - InitMoistDryB
Black (cell O3) specifies the label for the curve used in the chart's legend. It is copied from Sheet1 cell D4 in which the user replaced the color name with the actual ingredient name. The same thing applies to the other 14 columns (P through AC) that contain each isotherm curve's y-coordinate values.
Black (cells O4:O103), Blue (cells R4:R103), Green (cells T4:T103) and Red (cells W4:W103) contain the dependent (y) moisture values of the four ingredients calculated from each ingredient's moisture sorption isotherm equation using the Aw values in column N as the independent (x) variable of each equation.
Run the FindAw macro either by clicking the Find Aw* button or by selecting Macro> Macro... from Excel's Tools menu and then highlighting the macro named FindAw and clicking the Run button. The FindAw macro automates the use of the Goal Seek... command, which is also found on the Tools menu. The macro invokes the Goal Seek... command n+1 times for a model with n ingredients. Instead of running the FindAw macro, one could have used the Goal Seek... command manually for each of the five values sought in the demonstration mixture (AwStar and the four InitIngrAw values).
The accuracy attained by the Goal Seek procedure before it stops is set by choosing 'Preferences...' from the Tools menu then clicking on the 'Calculation' tab. The 'Iteration' checkbox has been checked and values of 100 for 'Maximum iterations:' and 0.0001 for 'Maximum change:' are used in the downloaded worksheet.
Excel water activity worksheet download page